Session 2005-1222
Excel™ Analysis of Combined Cycle Power Plant
Michael R. Maixner
United States Air Force Academy
A key issue in student design projects in thermodynamics is the necessity to
modify property values during iteration and/or redesign. This is particularly
true when dealing with two working fluids (e.g., air, water) in a combined
cycle. The necessity to manually ascertain these values at all points of the
cycle can inhibit the pedagogic purpose of the project: to allow students to
view how overall system parameters (efficiency, specific fuel consumption,
horsepower, etc.) may vary in response to changes in one or several input
parameters (turbine pressure ratio, ambient air temperature, barometric
pressure, cooling water temperature, boiler pressure, etc.).
A separate paper1 to be presented at this conference describes the details of
an Excel™ spreadsheet add-in that relieves the student of the laborious
updating of these property values as cycle modifications are made. This
paper presents the application of this Excel™ add-in to analyze a baseline
combined cycle plant (including cogeneration), and how various sensitivity
analyses and optimization problems may be used to enhance students’
understanding of the basic design. Additional plants that could be analyzed
are suggested.
“Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition
Copyright © 2005, American Society for Engineering Education”
Page 10.602.1
INTRODUCTION
Although they have learned the essential elements of various power plants and other
thermodynamics systems in their studies of thermodynamics, fluid mechanics, and heat transfer,
students frequently graduate without having analyzed a more complicated design which
incorporates elements from all of these disciplines. Cadets at the United States Air Force
Academy who elect to take a course in energy conversion are required to analyze an existing or
hypothetical plant which encompasses a higher degree of complexity including, perhaps,
elements of a combined cycle, cogeneration, etc. In the past, the complexity of this plant
precluded more than a rudimentary “first-pass” analysis, due in large part to the requirement to
read thermophysical properties from tables and insert them into the calculations—this left little
or no time for more meaningful design studies of the plant, “what-if” scenarios, parametric
studies, and the like. The development of the Thermal Fluids Toolbox by SpreadsheetWorld,
Inc. and its distribution as “freeware,” has removed much of the tedium from analyses such as
this, freeing the students to conduct more productive and instructive investigations of plant
design. Cadets have learned the rubrics of table-reading and interpolation in previous courses; at
this point in their education, they should be focusing more on the design aspects of thermal-fluid
systems. Most engineering students have a copy of Excel™ on their computers—this, coupled
with the “freeware” nature of the Thermal Fluids Toolbox (obtainable at
http://www.spreadsheetworld.com), makes it a computational resource which is readily available
to all engineers.
The Thermal Fluids Toolbox is described in more detail in another paper presented at this
meeting1; a brief synopsis of the application, however, is provided here. The “Toolbox
Overview” provided within the program describes this Excel™ “add-in:”
The Thermal Fluids Toolbox provides the capability to determine the
thermodynamic state of 40 common working fluids in four different sets of units.
The methods and equations utilized in this module are based on the computational
equations published in “Thermodynamic Properties in SI” by William C. Reynolds,
Stanford University, 1979. The toolbox functions may be accessed directly from
either the Excel™ worksheet or Visual Basic for Applications (after making a
reference to the toolbox). A graphic interface is also available which provides
instant access to the toolbox functions and provides the ability to insert these
functions onto a worksheet for subsequent analysis.
A sample screen shot of a relatively simple thermal-fluids system (a piston-cylinder
arrangement) is shown in Figure 1, taken from an ExcelTM spreadsheet distributed by
Page 10.602.2
Figure 1: Thermal Fluids Toolbox utilized in analysis of piston-cylinder arrangement.
“Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition
Copyright © 2005, American Society for Engineering Education”
SpreadsheetWorld as a demonstration of the capabilities of the Thermal Fluids Toolbox; in it, the
Thermal Fluids Toolbox dialog screen is open, showing how the arguments are inserted, and also
the calculated values of other thermophysical properties. While a simple example, repeated calls
to the Thermal Fluids Toolbox may be employed in significantly more complex systems.
The Course
The project is required as the major component of the grade in an energy conversion course.
While cadets have previously received instructin in the fundamental aspects of basic gas and
vapor cycles, they are here exposed to a more detailed treatment of each cycle, including cycle
modifications to improve efficiency. Additional topics are covered, to include solar, hydroelectric, fuel cells, nuclear, and the like. An additional course objective is that cadets be able to
interpret plant schematics, understand process engineering symbology, etc.; this is met through
continued reference to the plant schematic provided, and to modifications made to that diagram
by cadets in the course of their project.
Combined/Cogeneration Cycle
The first cycle on which Thermal Fluids Toolbox was chosen for use was a generic combined
cycle plant wherein a two-stage, twin-spool, intercooled gas turbine with reheat exhausts to a
steam generator; the superheated Rankine cycle incorporates an open and a closed feed heater,
and an economizer (Figure 2). Waste heat from the condenser was utilized to provide building
heat. The output of the plant was considered to be the electric power and building heat
produced. Each principal component in the plant has a prescribed isentropic efficiency, and an
effectiveness may be prescribed for each heat exchanger. Ambient/atmospheric conditions are
provided, as are economic data (fuel heating value, fuel price, loan information, etc.), and plant
staffing requirements (numbers of personnel, wages, benefits, etc.). A cursory glance at the plant
schematic reveals numerous points at which enthalpies, specific volumes, temperatures,
pressures, etc. must be prescribed or calculated—additionally, there are two different working
fluids: air and water.
1
Fuel
Combustor
4
LPC
8
5
HPC
HPT
LPT
Generator
PEG
2
6
3
Fuel
7
Reheater
Intercooler
Steam
Turbine
Generator
PES
9
Building HX
18
Pump 2
19
20
D
17
15
G
y18
y19
12
OFWH
C
E
16
F
11
14
13
CFWH
10
B
Condensate
Pump
Feed Pump
21
A
22
Figure 2: Combined cycle with cogeneration.
“Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition
Copyright © 2005, American Society for Engineering Education”
Page 10.602.3
T
Project Analysis
Cadets were arranged in teams of two, and each team was tasked with analyzing the plant at the
baseline (prescribed) conditions using an Excel™ spreadsheet template provided by the
instructor—whenever thermophysical properties were required, the Thermal Fluids Toolbox was
to be utilized, so that no manual data retrieval was required. The class lectures began with gas
turbines—cadets were introduced to the Thermal Fluids Toolbox during classroom sessions
where it was used on their laptop computers in the solution of homework problems. During
these lectures, cadets were provided the gas turbine portion of the project, relatively early in the
term. As cadets completed this portion of the project, they employed an ideal air standard, and
each team was free to compare answers with the instructor—the learning curve was not too
steep, and served to help reinforce cadets’ understanding of the air tables to which they had been
introduced in a previous course. This, and comparison of several data points with manual data
retrieval, helped to increase their confidence in the use of the Thermal Fluids Toolbox.
The next lecture topic included the Rankine cycle and various improvements (superheat, reheat,
regeneration, etc.); the second portion of the project made direct use of this material, and was
assigned at this point. Again, cadets completed this portion of the project using the baseline
conditions, and were able to compare the results to gain further confidence in the use of the
Thermal Fluids Toolbox.
The third portion of the project entailed an economic analysis of the entire plant, based upon the
baseline information provided.
“Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition
Copyright © 2005, American Society for Engineering Education”
Page 10.602.4
In the fourth phase of the project, all teams were required to perform parametric analyses on the
baseline plant; each team was also tasked to conduct a major plant design modification.
Analyses which all teams might be required to perform on the baseline plant could include:
• Parametric analyses on variations of power/specific fuel consumption/cost/efficiency/
utilization factor with
o Geographic location/seasonal variation in
Atmospheric pressure
Ambient air temperature
Cooling water temperature
o Gas turbine pressure ratio
o Boiler pressure
o Condenser pressure
• Effect of piping system
o Pressure drops
o Heat losses (resulting from damaged/inadequate/different types of insulation—
this tasking could be made as extensive as desired, incorporating
radiation/convection/critical radius calculations to the degree desired).
• Sensitivity of plant costs to
o Refinancing of loan
o Pay raises—how much of a percent pay raise could we allow before we break the
bank
o Change in cost of fuel
•
o Condensate depression
Optimal steam plant extraction pressures and flowrates (required incorporation of
Excel™’s Solver feature—Goal Seeker might also be employed in various aspects of the
project). The idea for this particular calculation was obtained from a design project
employed at California State University at Northridge by Dr. Larry Caretto.
Each team was also tasked with performing a somewhat more significant modification to the
plant. Such possible modifications included, but were not limited to:
o Gas turbine
Regeneration in lieu of reheat
Changing number of stages
Effect of removing intercooling
o Steam
Remove superheat
Add reheat
Add more stages of regeneration
When cadets were provided the spreadsheet template, all pertinent cells had been named by the
instructor—this greatly facilitated troubleshooting and grading. Also, when cadets turned in
their completed projects, two versions of the spreadsheet were required as “deliverables:” one
with a numerical result in each cell, and another with equations fully displayed—both versions
were also required to depict row and column headings and grid lines to facilitate troubleshooting
by the cadets and grading by the instructor.
Cadet Feedback
The end-of-term feedback from cadets was not available when this manuscript was drafted.
In general, however, cadets invariably greeted the Thermal Fluids Toolbox with a great deal of
enthusiasm—the most common comment made was “why didn’t we use this when we were first
introduced to the property tables?!” Of course, the response revolved about ensuring that they
had adequate familiarity with the tables, knew how to perform interpolation, etc. The cadets,
relieved of the tedium associated with “property look-ups,” were then able to devote more time
to understanding underlying cycle principles and to investigate the more global aspects of
parametric variations.
“Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition
Copyright © 2005, American Society for Engineering Education”
Page 10.602.5
Problems
No application or add-in will be perfect. As with any spreadsheet, appropriate conversion factors
must be incorporated whenever required. Although Excel™ does provide some elementary unit
conversions, it is still incumbent upon the user to employ these correctly and to accomplish those
for which no “library” conversions are provided. Mathcad™, which handles units quite nicely, is
particularly attractive in this regard. Additionally, Mathcad™ markets a product which provides
a similar property “look-up” capability (at a cost, of course); there are a handful of freeware
applications written to perform similar functions, but they are not nearly as easy to use as is the
Thermal Fluids Toolbox. Finally, Mathcad™ is not nearly as widespread in adoption as is a basic
spreadsheet—it is difficult to find a workplace where ExcelTM has not been adopted in some
fashion.
An aspect of the add-in which might prove to be a problem is that the datum for various working
fluids may differ from those provided in tabular presentation of such data. Normally, this is not
usually an issue, since engineers are usually interested in differences of properties (most
commonly, enthalpy). The Thermal Fluids Toolbox does, however, incorporate a feature which
allows specification of a datum from which all other calculated properties may be referenced, so
that Thermal Fluids Toolbox output may be made to match values interpolated from tabulated
data.
The Thermal Fluids Toolbox provides an error indication when operating at or near critical or
saturation conditions when dealing with a two-phase fluid; cadets encountered this difficulty
when investigating the effects of condensate depression on plant performance. This issue may
be circumvented by specifying that the quality is either saturated liquid or saturated vapor, or by
incorporating appropriate logic checks within the spreadsheet cell. Additionally, when cadets
performed Otto and diesel cycle analyses as homework assignments (not a part of the project
analysis, though), it was discovered that the Thermal Fluids Toolbox did not provide property
values for air at temperatures on the order of 4000°F; discussions with the authors of Thermal
Fluids Toolbox indicated that this was out of the range of the computational equations employed
within the program.
Finally, the accuracy of the results provided by the Thermal Fluids Toolbox is not perfect.
Since, as stated in the documentation which accompanies the Toolbox, the results are dependent
upon “computational equations,” they are certain to contain errors. These errors are small,
however, and in no way detract from the educational benefit afforded by the Toolbox. Other,
more robust products are available to provide the desired degree of accuracy…..at a cost, of
course.
Future Uses
The Thermal Fluids Toolbox is an extremely useful tool in the analysis of many process
engineering systems. It is envisioned that it will be applied to various other systems in future
course offerings—internal combustion engine simulation, compressed air energy storage plants,
and others come to mind as possible candidates.
Additionally, the Toolbox has also been used for instructor templates in conjunction with
designing and testing thermal-fluids labs at the Air Force Academy, where data may be entered
in appropriate spreadsheet cells—once entered, thermophysical properties are immediately
evaluated, allowing further calculations to execute immediately. Real-time checks on the
laboratory and associated data reduction allow instantaneous feedback. In an elementary
thermodynamics course, cadets could be tasked to construct temperature-entropy or Mollier
diagrams with the Thermal Fluids Toolbox; by actually “building” these (and other) diagrams,
the cadets’ understanding of the tabular and graphical relationships between thermophysical
properties will be greatly enhanced. In a more advanced course, cadets could actually construct
much of a psychrometric chart.
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“Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition
Copyright © 2005, American Society for Engineering Education”
Conclusion
Incorporation of the Thermal Fluids Toolbox add-in to Excel™ allows students to focus on the
more important aspects of a design, rather than becoming mired in the minutiae of property
“look-ups.” With the proper introduction and orientation to the Toolbox, cadets can gain
familiarity and confidence in its use, which will ultimately facilitate their understanding of the
functioning of the thermal system(s) under consideration. The Toolbox may be employed for a
variety of working fluids, and in many process engineering applications1. In particular, it is
useful in the analysis of combined cycle and/or cogeneration plants.
Acknowledgements
Thanks are due to Mr. David McDaniel, CPhil, of the Department of Aeronautics at the United
States Air Force Academy, and to Dr. Larry Caretto of the Mechanical Engineering Department
at the California State University at Northridge for their assistance in troubleshooting various
aspects of the Thermal Fluids Toolbox during the author’s work. Mr. McDaniel also graciously
agreed to review and comment on an early draft of this manuscript.
Disclaimer
The views expressed are those of the author and do not reflect the official policy or position of
the US Air Force, Department of Defense or the US Government.
1
Caretto, L, McDaniel, D., and Mincer, T., Spreadsheet Calculations of Thermodynamic Properties, 2005 ASEE
Annual Conference & Exposition, paper 2005-297.
MICHAEL R. MAIXNER
Received his B.S. from the United States Naval Academy, S.M.M.E. and Ocean Engineer degrees from MIT, and
Ph.D. (mechanical engineering) from the Naval Postgraduate School. Following 12 years as a line officer and 13
years as an engineering duty officer in the USN, he taught at Maine Maritime Academy for five years before
accepting a position in the Department of Engineering Mechanics at USAFA.
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“Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition
Copyright © 2005, American Society for Engineering Education”