Energy Based Systems Modeling of a
Steam Turbine System
ME 6105- Modeling and Simulation in Design
Colin Riley and James Spence
11/5/2011
Task 1: Goals and Problems Domain of the Steam Turbine System
Steam turbines serve as the most common power generation technology of the modern world. By
converting heat energy into rotational work, steam turbines are the source of approximately 90% of
electrical power used in the world today. The goal of this project is to maximize the profitability of a
steam turbine system by optimizing the power generation cycle.
The goal of this project is to maximize the profitability of the power generation cycle by maximizing the
efficiency of the steam turbine system. Efficiency will be maximized when the energy output of the
steam turbine is maximized, while the amount of fuel used by the boiler and the cost of equipment is
minimized. Our original goal was to accomplish this by modeling a basic steam turbine system and
witness the affect of introducing different configurations of feedwater heaters into the system;
however, after realizing the complexity of creating multiple configurations of the system, we have
decided to focus on just one configuration of a basic steam turbine system. We will characterize the
system by varying four design parameters: the initial temperature of the steam turbine, the condenser
pressure, the pressure at which steam is removed for reheat, and the temperature of reheated steam
entering the system. These parameters will help us optimize the system, and maximize profit by
maximizing energy output while minimizing energy input.
Design Scenarios:
As stated above, we will vary four design parameters to characterize the system: the initial temperature
of the steam turbine, the condenser pressure, the pressure at which steam is removed for reheat, and
the temperature of reheated steam entering the system. We will design a high/low condition for each of
these parameters while holding the other parameters constant. This will be used, in the end, to find an
optimal state of the system by using the individual parameter relationship demonstrated.
Table 1 below shows how the design parameters will be varied:
Table 1: Design Parameter Variations
Initial Steam
Temperature
Condenser
Pressure
Steam Reheat
Pressure
Reheat Steam
Temperature
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
500 C
550 C
500 C
500 C
500 C
0.1 bar
0.1 bar
0.2 bar
0.1 bar
0.1 bar
55 bar
55 bar
55 bar
40 bar
55 bar
500 C
500 C
500 C
500 C
550 C
We will characterize the overall effect of varying each parameter by looking at the total energy output of
the turbine, the energy input required by the boiler and reheater, and the efficiency determined by the
ratio between the two values.
Table 2 below provides a brief summary of the expected results from varying each parameter.
2
Table 2: Design Parameter Effect Expectations
Initial Steam
Temperature
Condenser
Pressure
Steam Reheat
Pressure
Reheat Steam
Temperature
Turbine Power
Output
As temperature
increases, output
increases
As pressure
decreases, output
increases
As pressure
decreases, output
increases
As temperature
increases, output
increases
Cycle Heat Input
As temperature
increases, heat
input increases
As pressure
decreases, heat
input increases
As pressure
decreases, heat
input increases
As temperature
increases, heat
input increases
While these results are expected based on basic thermodynamic relationships, the sensitivity of each of
these parameters is what will determine the effect on overall cycle efficiency. This project will look to
quantify the effects of varying the above design parameters and use these relationships to identify
desired steam turbine cycle conditions.
Task 2: System and Simulation Specification
The steam turbine system is comprised of a boiler, steam turbine steampath with reheater, a condenser,
two feed pumps, an optional feedwater heater, and a simple generator. See Figure 1 for a depiction of
the system.
Figure 1: Steam Turbine Cycle Flow Diagram
As seen above, the components interact in a closed water cycle, with only heat and work being
transferred in and out of the system. Shaft power from the steam turbine will be converted to electrical
3
power by a generator, which is not included in this project’s scope. Each component defines a key
aspect of operation that will influence the behavior of other components and the overall system.
During steady-state operation, feedwater is pumped into a boiler/steam generator at a given pressure
and temperature. The boiler will produce steam at the pressure determined by the feedwater, often
superheating the steam to a temperature above the steam's saturation temperature. The boiler
determines the temperature at which steam will enter the turbine.
The steampath next expands the steam produced as a specified temperature and pressure down to a
pressure specified by the condenser. The steampath component of the steam turbine produces the
shaft work that will eventually be turned into electricity by an associated generator.
Steam turbines with a reheater take partially expanded steam exiting a high pressure section of the
steam turbine, heat it to a higher superheated temperature, and readmit the steam to the steampath.
The reheater design determines the temperature of the steam reentering the turbine and the size of the
lower pressure steampath determines the pressure at which the steam is reheated.
The condenser of the steam turbine cycle determines the pressure to which the steam will ultimately
expand. The condenser removes the heat from the expanded steam until all steam is condensed into
water. Water at condenser pressure is then sent to the feedwater pumps to repeat the cycle.
Overall Model Goal:
The model developed for this project will concentrate solely on the thermodynamic operation of the
steam turbine. The physical aspects of the steam turbine system, such as boiler size and material, will
not be considered. The design parameters investigated by this project are all thermodynamic variables,
so rotational and heat transfer energy system parameters do not need to be modeled beyond
simplifying assumptions.
Steam turbines spend the majority of their operating lives at steady-state conditions. As such, steam
turbines are designed to perform best at defined steady-state or set of defined steady states. The model
developed for this investigation will not model transient operation of the system. It will instead look to
characterize the effects of steady-state design parameters on steam turbine output and performance.
The working fluid in a turbine is defined thermodynamically by two intensive variables and one
extensive variable. As long as these three parameters are defined at every point of operation, the
working fluid will be fully defined. Each of this project's steam turbine model components ensures that
the change in water or steam enthalpy and pressure during operation is accurately defined, while the
flow rate of water and steam through the system is maintained at a constant rate.
4
Individual Component Overview:
Boiler:
Figure 2: Boiler Operation Representation
Figure 2 shows a basic representation of boiler operation in a steam turbine cycle. Water enters the
boiler as sub-cooled liquid, brought to saturation temperature, and boiled in a vessel at a controlled
pressure near the pressure of the sub-cooled liquid. This saturated steam is then sent through a heat
exchanger, often using the same heat source used in boiling at a hot medium, and the steam is
superheated to a desired temperature. Boilers will experience a pressure drop from the pressure of the
sub-cooled liquid during operation, both from the boiler vessel and the superheater piping and valves.
Steam Turbine Model Application:
While the physical design of the boiler and the type of heat input to the system determine the
temperature of the steam leaving the boiler in real world applications, this project's model of a steam
thermodynamic cycle is only concerned with the effects the boiler will have on the pressure and
enthalpy of the working fluid.
The steam inlet temperature to the steampath is a design parameter of interest for this investigation. To
control this design parameter, the enthalpy rise of the boiler should be easy to manipulate.
This component should thus model an enthalpy rise and pressure drop in the working fluid. Both of
these values should be user inputs to the system.
5
Assumptions:
The boiler component of this model will assume:



The type of heat input used is irrelevant to the model being developed.
Pressure drop through the boiler is approximately 10% [2].
The heat input to the system is equal to the heat transferred to the working fluid.
Steampath:
Figure 3: Steam Turbine Steam Path Overview
Figure 3 illustrates the overall thermodynamic operation of a three pressure steam turbine. Steam from
the boiler (designated as the “heater” in Figure 3) is admitted at a specified pressure and enthalpy to the
high pressure section of the steam turbine. After expanding to an intermediary pressure, steam is
removed from the steampath and passed through a steam reheater (called the “intermediate heater” in
Figure 3). After a second expansion, steam is passed to a dual flow low pressure section, through which
the steam is expanded to a pressure specified by a connected condenser.
Steam turbines use a traditional stator/rotor configuration to produce power. By expanding through a
series of stages increasing in physical area, steam is directed by a stationary “nozzle” component to a
rotating “bucket” component in each stage to produce shaft power. This shaft power is used by a
generator to produce electrical power.
The physical design and dimensions of the steampath, such as number of stages, bucket length, and the
inner rotor diameter will determine a thermodynamic efficiency for the expansion process. A higher
thermodynamic efficiency translates to lower entropy generation rates and a greater amount of work
extracted from the steam expansion.
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Steam Turbine Model Application:
In the basic thermodynamic model of a steam turbine steampath approached by this project, two
variables will have a meaningful impact on operation: the steampath efficiency and the pressure at
which steam is extracted from the steampath and sent through the reheater.
The Modelica ThermoPower library currently has examples of several steam turbine configurations that
are leveraged for this investigation. Steampath efficiency in these models is a direct user input that can
be easily manipulated.
Modelica steam turbine examples use an active area variable to define the sizing of the steampath at
each section. Under a constant flow rate, this active area will set the pressure at the entrance to each
steam turbine section. By adjusting the active area at the inlet of the intermediate pressure section, the
pressure at which steam is extracted for reheat can be directly manipulated at a given flow rate.
Assumptions:
The steam turbine steampath component of this model will assume:



Steampath thermodynamic efficiency is approximately 90% [2].
Steam leakages from the steampath are negligible.
Heat loss while passing through the steampath is negligible.
Reheater:
In most steam turbine applications, the reheater functions like the superheating section of the boiler
described above. Heat Recovery Steam Generators in combined cycle applications actually use the same
equipment and heat source to produce the main steam and reheat partially expanded steam. Like
boilers, reheaters will experience a pressure drop as the steam moves through the equipment.
Steam Turbine Model Application:
As in the boiler, this project's model of a steam thermodynamic cycle is only concerned with the effects
the boiler will have on the pressure and enthalpy of the working fluid.
The reheat temperature reentering the steampath is a design parameter of interest for this
investigation. To control this design parameter, the enthalpy rise of the reheater should be easy to
manipulate.
This component should thus model an enthalpy rise and pressure drop in the working fluid. Both of
these values should be user inputs to the system. The boiler and reheater should actually be able to use
the same component in the steam turbine model.
7
Assumptions:
The reheater component of this model will assume:



The type of heat input used is irrelevant to the model being developed.
Pressure drop through the reheater is approximately 10% [2].
The heat input to the system is equal to the heat transferred to the working fluid.
Condenser:
Figure 4: Condenser Operation Representation
Figure 4 shows a representation of condenser operation in a steam turbine cycle. Steam condensers
normally operate at a vacuum determined by the temperature of the cooling water. Lower cooling
water temperatures will have a lower condenser pressure. The cooling water absorbs heat from the
exhaust steam, normally already at or near the saturation temperature, until the steam is fully
condensed. The condensate is then drained from the condenser and sent to the feedwater pump
system.
Steam Turbine Model Application:
In designing a steam turbine plant, condenser sizing and design to ensure a steady level of condensate
during operation and minimize the turbine exhaust pressure. In a thermodynamic model, however, a
specified exhaust pressure is the only design parameter of interest in the condenser component.
The Modelica ThermoPower library has a condenser component that allows the user to directly specify
an exhaust pressure. This component will therefore specify the pressure to which the steam turbine will
expand, provide the heat removal necessary to fully condense the exhaust steam, and define the
enthalpy at which the condensate leaves the condenser.
Assumptions:
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The condenser component of this model will assume:


Exhaust pressure will be a steady state value for a given condenser design.
No pressure drop will be observed in the condensate from condenser to feedwater pump.
Feedwater Pumps:
The feedwater pump arrangement in a steam turbine cycle is responsible for increasing the pressure of
the water feeding the boiler from condenser pressure to boiler inlet pressure. The power used to
perform this operation raises the enthalpy of the feedwater along with the pressure.
Steam Turbine Model Application:
In a thermodynamic analysis of a steam turbine cycle, pump operation is often considered separately
from the rest of the cycle’s design. The goal of the feedwater pump arrangement is simply to get the
condensate from condenser pressure to the desired boiler pressure.
The Modelica ThermoPower library has a basic pump component. This component, however, provides a
more defined physical model than usable by this project. Changing the flow from a nominal pump design
value will change the pump discharge pressure. This component should be modified slightly so that the
discharge pressure can be explicitly defined rather than calculated from a specific pump design. All other
relevant pump calculations will be performed as modeled in the example component.
Assumptions:
The feedwater pump component of this model will assume:


Pump discharge pressure is a value directly defined by the user, not a value based off a set
pump design.
No heat or mass will be lost through pump operation.
System Assumptions:
This system will model the thermodynamic cycle the working fluid of a steam turbine (water/steam)
experiences. From this cycle, required heat input, heat rejection, and pump power will be determined,
although the processes of heat transfer and shaft power to the pump will not be modeled in Modelica.
Likewise, the steam turbine shaft power output will be determined, but its conversion to electrical
power will not be modeled.
The following system assumptions will be made:

Other than specified heat inputs in the boiler and reheater and heat rejection in the condenser,
the steam turbine operates adiabatically.
9




System pressure drops are only observed in the boiler and reheater components. No valve or
line pressure drops in the rest of the steam turbine system will be modeled.
The generator and pump power source are outside of the project scope. The shaft power from
the steam turbine and the shaft power required of the pump will be provided as a project
output.
Likewise, the boiler/reheater heat source and condenser cooling system is outside of the project
scope. The model will provide values necessary for these systems.
No steam is lost in the steam turbine cycle.
Assumed cycle parameters will be discussed in Task 5.
10
Task 3: Create the Steam Turbine Model
Figure 5 below shows a screenshot of the Modelica model used to develop the steam turbine cycle with
reheat for this project.
Figure 5: Steam Turbine Model
The Modelica model consists of five major steam turbine cycle components, two Modelica blocks to
define the system, and several specified constant input values. This section will discuss each component
individually, identify the inputs required, and detail the connections between components.
11
Boiler/Reheater:
Figure 6 below shows a screenshot of the Modelica model used to perform calculations of the boiler and
reheater in the steam cycle.
Figure 6: Boiler/Reheater Modelica Model
The boiler/reheater model is simple in construction. Water/steam enters the model from the upper fluid
connection and experiences a change in thermodynamic state via a pressure drop and an enthalpy
increase as specified by real input connectors dp and dh respectively. The working fluid then exits the
model through the lower fluid connection at the thermodynamic state determined by the pressure drop
and enthalpy rise user inputs. No other values are used in the boiler/reheater model, as the
boiler/reheater is treated as a black box to change thermodynamic state in this investigation.
12
Condenser:
Figure 7 below shows a screenshot of the Modelica model used to perform calculations of the
condenser in the steam cycle.
Figure 7: Condenser Modelica Model
The condenser component used in this project's steam turbine model is the CondenserPreP component
from the ThermoPower Modelica library. This component has parameter inputs of condenser pressure,
condenser fluid slide total volume, and the initial liquid water volume. Since the geometry of the
condenser is not specified in this investigation, the volumetric parameters are irrelevant to the study.
Exhaust steam from the turbine enters the condenser at the upper fluid connection and condensate
leaves the condenser at the lower fluid connection. The condenser component condenses the steam to
saturated liquid, determining the amount of heat removal necessary to run the condenser effectively.
13
Feedwater Pump:
Figure 8 below shows a screenshot of the Modelica model used to perform calculations of the feedwater
pump in the steam cycle.
Figure 8: Feedwater Pump Model
The feedwater pump component used in this project's steam turbine model is a slightly modified version
of the Pump component from the ThermoPower Modelica library. While the original pump component
calculates the head based on the flow rate through the pump and specified pump nominal conditions,
the modified model allows the user to directly input the pressure increase across the pump. By leaving
all other calculations the same and bypassing the need for a nominal pump design, all other relevant
pump operation value calculations remain accurate.
This component has parameter inputs of pressure increase, pumps in parallel, rotational speed, pump
internal volume, nominal liquid density, and power calculation method. The pump component also has
an initialization page and ability to designate the component as a steady state design. As the system
modeled is steady state and the geometry of the pump is not considered, the only relevant parameter
inputs are pressure increase, nominal liquid density (assumed 1000 kg/m3 for water), and rotational
speed. Rotational speed is defined by a user input block.
The pump component will characterize the discharge water pressure and enthalpy, as well as calculate
the power required to operate the pump.
14
Reheat Steampath:
Figure 9 below shows a screenshot of the Modelica model used to perform calculations of the
steampath with reheater provisions in the steam cycle.
Figure 9: Reheat Steampath Model
The reheat steampath component used in this project's steam turbine model is the ST3LRh_base
component from the ThermoPower Modelica library. The most complex component of the steam
turbine cycle model, the reheat steampath component has fluid connections that allow for inlet steam
to enter, expand through the ST_HP section, exit for reheat, reenter and expand through the ST_IP
section, combine with an LP admission flow before expanding through the ST_LP section, and finally
leave the model through a connection to the condenser.
15
The parameter inputs of the reheat steampath component are a series of nominal flow rates and
pressures at the inlet of each steam turbine section, the pressure of the condenser, the internal volume
of the LP mixer, the active area flow coefficient at the inlet of each turbine section, the mechanical
efficiency of each section, and the isentropic efficiency of each section. The initialization page of the
component has an array of initial values within the steam turbine and the ability to define steady state
operation.
The reheat steampath in this project's cycle does not utilize LP admission flow, nor does the steampath
operate at conditions other than steady state. Sensors and actuators are also not use in this model.
Therefore, the parameters of interest in the steampath component are the condenser pressure, the
active area flow coefficients to set the steam turbine inlet pressures, and the mechanical/isentropic
efficiencies to identify the steam expansion line and power output from the steam turbine.
Model Overview:
To successfully run, the steam turbine model requires two blocks from the ThermoPower library to fully
define the system, the ThroughFlow block and the PrescribedSpeed block. Through user-defined input,
these components set the mass flow rate and rotational speed of the steam turbine (assumed to be
3600 RPM, or designed for a 60 Hz power grid).
Since it is assumed no working fluid mass losses are present in the system, the value specified by
ThroughFlow sets the steam flow rate of the steam turbine cycle. Combined with the active area
definitions in the reheat steam turbine component, the pressure of the condenser, the discharge
pressure of the pump, and the pressure and enthalpy changes in the boiler and reheater, the user is able
to fully define the steam turbine thermodynamic cycle.
16
Task 4: Verify the Model Works
Boiler Model:
Initial_Pressure
Initial_Enthalpy
Mass_Flow _?
k=12000000
k=900000
k=100
h
p0
P
throughW
sourceP
Drop_Pressure
k=2100000
Drop_Enthalpy
boiler
k=100000
Final_Pressure
k=10000000
h
p0
P
sinkP
Figure 10: Boiler Component Model
To test the boiler component, we send a flow of water through the boiler at a specific pressure (120
bar), enthalpy (900,000 J/kg), and mass flow rate (100 kg/s). In the boiler itself, we control how much
the boiler will heat the water by specifying the change in pressure (21 bar) and enthalpy (100,000 J/kg).
So, if the model is working correctly, the water output will be at a higher pressure and enthalpy than the
water input. Figure 11 shows the results of the test which clearly indicate that when the component
model is run, the input pressure (120 bar) is higher than the output pressure (100 bar) by the amount
specified, and the input enthalpy (900,000 J/kg) is less than the output enthalpy (1,000,000 J/kg) by the
amount specified (100,000 J/kg). These results are the expected behavior of a boiler: it heats water to
steam which will increase the enthalpy and decrease the pressure.
17
125
120
115
110
105
100
95
0.0
1.2E5
boiler.waterIn.p
0.1
boiler.waterOut.p
0.2
0.3
boiler.waterIn.hAB
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.6
0.7
0.8
0.9
1.0
boiler.waterOut.hAB
8.0E4
4.0E4
0.0E0
0.0
0.1
0.2
0.3
0.4
0.5
Figure 11: Boiler Test Results
Turbine Model:
Pressure_In
Mass_Flow _?
k=100
h
p0
P
k=100
sourceP
throughW
Pressure_Out
constantSpeed
k=2000000
steamTurbineUnit
Time_Constant
h
p0
P
sinkP
k=1
Figure 12: Turbine Component Model
18
We test the turbine model in much the same way as the boiler model: send steam through the turbine
component at a specified mass flow rate (100 kg/s) and pressure (100 bar). When this happens, we
should see the shaft of the turbine spin, power generated, and the output pressure to be lower than the
input pressure. Figure 13 shows the shaft rotation angle increasing, and that the shaft has an angular
velocity of 377 rad/s or 3600 rpm; these show that the turbine model is accepting the steam inputs and
that it is using that to generate power. Figure 14 shows that the high pressure turbine is generating 20
MW of power and the low pressure turbine is generating 8.8 MW. Additionally, the enthalpy of the
steam going into the turbine is greater than the enthalpy going out, as seen in Figure 15, and the
pressure going into the turbine is greater than the pressure going out, as seen in Figure 16. Now, if we
decrease the input mass flow rate, or the rate at which steam is entering the turbine, the total power
output should go down; Figure 17 shows that when we lower the mass flow rate to 25 kg/s, the power
output of the high pressure turbine is decreased to 800 kW as opposed to 20 MW when the mass flow
rate was at 100. So, the model is working as we would expect.
2.4E4
steamTurbineUnit.phi [deg]
steamTurbineUnit.der(phi) [rad/s]
2.0E4
1.6E4
1.2E4
8.0E3
4.0E3
0.0E0
0.00
0.25
0.50
0.75
1.00
Figure 13: Turbine Rotation
19
2.2E7
steamTurbineUnit.P_HP
steamTurbineUnit.P_LP
2.0E7
1.8E7
1.6E7
1.4E7
1.2E7
1.0E7
8.0E6
0.00
0.25
0.50
0.75
1.00
Figure 14: Turbine Power Output
steamTurbineUnit.hout
steamTurbineUnit.hin
3.00E6
2.96E6
2.92E6
2.88E6
2.84E6
2.80E6
2.76E6
2.72E6
2.68E6
0.00
0.25
0.50
0.75
1.00
Figure 15: Turbine Enthalpy
20
110
steamTurbineUnit.inlet.p
steamTurbineUnit.outlet.p
100
90
80
70
60
50
40
30
20
10
0.00
0.25
0.50
0.75
1.00
Figure 16: Turbine Pressure In/Out
8.5E5
steamTurbineUnit.P_HP
steamTurbineUnit.P_LP
8.0E5
7.5E5
7.0E5
6.5E5
6.0E5
5.5E5
5.0E5
4.5E5
4.0E5
3.5E5
3.0E5
0.00
0.25
0.50
0.75
1.00
Figure 17: Low Mass Flow Rate
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Condenser Model:
Pressure_In
Enthalpy_In
Mass_Flow _?
k=15000
k=2000000
k=100
h
p0
P
sourceP
throughW
Condenser
Figure 18: Condenser Model
The condenser works in the opposite manner as the boiler, so when we test this component, we should
see the input enthalpy be greater than the output enthalpy, as seen in Figure 19.
22
2.2E6
condenserPreP.steamIn.hBA
condenserPreP.steamIn.hAB
2.0E6
1.8E6
1.6E6
1.4E6
1.2E6
1.0E6
8.0E5
6.0E5
4.0E5
2.0E5
0.0E0
0.00
0.25
0.50
0.75
1.00
Figure 19: Condenser Enthalpy
To further test this component, if we decrease the mass flow rate, we should see the rate at which
steam turns to liquid drop. In the tested configuration, the rate at which the steam mass was changing is
-9 grams/s; if we drop the mass flow rate to 25 kg/s, that rate drops to -2.2 grams/s (as seen in Figure
20). So, this component is working as expected.
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condenserPreP.der(Mv)
-0.002
condenserPreP.der(Mv)
-0.003
-0.004
-0.005
-0.006
-0.007
-0.008
-0.009
0.00
0.25
0.50
0.75
1.00
Figure 20: Condenser Steam to Liquid Rate Test
Pump Model:
PressureOut
Pres?
Entha?
k=10?
k=90?
RotationalSpeed
k=10000000
k=100
h
p0
throughW
h
p0
n
P
P
sinkP
sourceP
pump
MassFlow Rate
k=100
Figure 21: Pump Model
The Pump model is very simple, water flows in and water flows out at a rate specified by the rotational
speed of the pump and the amount of water flowing into the pump. When the mass flow rate (going
into the pump) is 100 kg/s, the volume flow rate (out of the pump) is 36 m3/s. If we increase the mass
flow rate to 10000 kg/s, the volume flow rate increases to 3600 m3/s (see Figure 22). That means that
when the pump has access to more water, it pumps more. We should also expect to see an increase in
pressure and enthalpy when water flows through the pump. Figure 23 shows the increase in enthalpy
24
between the incoming and outgoing fluid, and Figure 24 shows the increase in pressure. So, the model is
working as expected.
4000
pump.q
pump.q
3500
3000
2500
2000
1500
1000
500
0
-500
0.00
0.25
0.50
0.75
1.00
Figure 22: Pump Volume Flow Rate Test
5.5E6
pump.hin
pump.hout
5.0E6
4.5E6
4.0E6
3.5E6
3.0E6
2.5E6
2.0E6
1.5E6
1.0E6
5.0E5
0.00
0.25
0.50
0.75
1.00
Figure 23: Pump Enthalpy Test
25
pump.pin_start
pump.pout_start
100
80
60
40
20
0
0.00
0.25
0.50
0.75
1.00
Figure 24: Pump Pressure Test
Task 5: Experimentation and Interpretation
Since the current model does not allow for a direct temperature definition, the values in Table 3 were
used to meet the conditions specified in Task 1.
Table 3: Full Steam Turbine System Test Parameters
Initial Steam
Enthalpy
Boiler Absolute
Pressure Drop
Reheat Absolute
Pressure Drop
Reheat Steam
Enthalpy
Pump Outlet
Pressure
Trial 1
3375
kJ/kg
Trial 2
3500
kJ/kg
Trial 3
3375
kJ/kg
Trial 4
3375
kJ/kg
Trial 5
3375
kJ/kg
10 bar
10 bar
10 bar
10 bar
10 bar
5.5 bar
5.5 bar
5.5 bar
4.0 bar
5.5 bar
3428
kJ/kg
3428
kJ/kg
3428
kJ/kg
3445
kJ/kg
3544
kJ/kg
110 bar
110 bar
110 bar
110 bar
110 bar
These conditions were met by modifying: the boiler enthalpy drop, reheater enthalpy drop, condenser
pressure, reheat pressure drop, and the pump outlet pressure. In each trial run, the simulation took
about six seconds to reach a steady state because of the multiple components interacting with pressure
and enthalpy. After this initial period, the system was clearly at a steady state where we could take
measurements of the heat input (enthalpy drop of the boiler and reheater multiplied by the mass flow
rate) and the mechanical power output (the sum of the individual turbine outputs). Then, to get a simple
efficiency, we divided mechanical power output by the heat input. The results of the experiments are
shown in Table 4.
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Table 4: System Experiment Results
Heat Input
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
317810 kW
341583 kW
340417 kW
342080 kW
351457 kW
Mechanical Power
Output
112860.5 kW
128348.8 kW
122526.3 kW
127814.4 kW
132863.9 kW
Overall Efficiency
35.5%
37.6%
36.0%
37.4%
37.8%
What these results show is that Trials 2, 4, and 5 were the most efficient configurations of the system.
Trials 2 and 5 set a high enthalpy (temperature) output for the boiler and reheater, while trial 4 did not
reduce the pressure of the reheated steam as much as the other trials. What this seems to show is that
the steam turbine system is most efficient at higher temperatures and pressures.
Task 6: Lessons Learned
This assignment highlighted several difficulties inherent in modeling a complex energy-based system.
We discovered that the first priority in modeling a system should be to understand your modeling tool’s
capability. Our initial project plan from Assignment 2 was to create several different steam turbine cycle
feedwater heater configurations. Yet within the object-oriented world of Modelica, this would require
creating a completely new model for each desired configuration. While possible to do, this strategy
opens the system to unintended errors and inconsistencies between models. We ultimately decided to
focus on parameters that can be directly manipulated in one configuration of model components.
We also learned that as the scope of your system of interest increases, more aggressive simplifying
assumptions need to be made to keep the model manageable. This project looks to define and make
system decisions on the entire thermodynamic cycle of a steam turbine. While a model that provides a
physical accounting of every single component in the steam turbine cycle will be more accurate, this
would require a vast knowledge of the physical characteristics of each piece of equipment in the steam
turbine cycle. The alternative approach, taken for this project, is to turn each component into a “black
box”, for which only parameters of interest such as pressure and enthalpy are reasonably accurate.
If we were to create another simulation model with the intended complexity of this class, we could
consider choosing a specific component within the steam turbine cycle, such as the condenser, and
focus on the physical design of that component. While this model would not be able to reach the overall
conclusions about power output and heat input in a steam turbine cycle, more specific design decisions
could be made.
These lessons learned point to another: keep the scope of your project as narrow as possible. Each
component adds a level of complexity that may be too much to handle. Each component not only needs
to be designed, but also needs to be tested, understood completely, and modified to suit your individual
needs. We had to lessen the scope in several instances to get the project off the ground.
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Related to this, is to not add complexity unless the model is up and running. Choosing to add multiple
components simultaneously only increases frustration and confusion and lengthened the time to
completing the model.
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