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  “Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition Copyright © 2005, American Society for Engineering Education” 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 paper 1  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. 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 P  a g e1  0 . 6  0 2 .1   “Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition Copyright © 2005, American Society for Engineering Education” 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 meeting 1 ; 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 Excel TM  spreadsheet distributed by   Figure 1: Thermal Fluids Toolbox utilized in analysis of piston-cylinder arrangement. P  a g e1  0 . 6  0 2 .2   “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, hydro-electric, 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.   Intercooler Combustor Reheater LPC HPT LPTHPC1234 5FuelFuel76FG Pump 2 1615 CFWH TOFWH Generator P ES SteamTurbineGenerator P EG CondensatePumpFeed PumpBuilding HX 181719 2021221413121110DCEBA98 y 18  y 19   Figure 2: Combined cycle with cogeneration. P  a g e1  0 . 6  0 2 . 3   “Proceedings of the 2005 American Society for Engineering Education Annual Conference & Exposition Copyright © 2005, American Society for Engineering Education” 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. 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: ã   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 ã   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). ã   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 P  a g e1  0 . 6  0 2 .4 

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Oct 7, 2019

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Oct 7, 2019
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