5. The ThermoFluid Library
5.10 ThermoFluid Applications
Chapter 5. The ThermoFluid Library
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Molefraction [1]
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Figure 5.14 Molar fractions of the principal reactants and products
trol in steam power plants. It uses a ThreePort model for the mixing of subcooled water and superheated steam. It uses propagation of record parameter for propagating parameters from the top level into the compo-nents. This example uses only available components from theThermoFluid library.
5.10 ThermoFluid Applications
(a) Top Level and Control (b) Internal Structure
Figure 5.15 Schematic of a Steam Power Plant Attemperator with a PID-Controller.
• Combined heat and power micro turbine systems.
• Distribution of Cl2 and H2 in a large chemical plant.
• Pasteurization by steam injection in food processing plants.
In many cases the use of the library was exactly as intended: the base classes were used extensively to create a customized model library for a particular application. Some of the projects got along with the ready-to-use models in the library, other ones had to add a substantial modeling effort in order to achieve their goal, but all of them have in common that all dynamic parts of the model are built by inheriting from the library classes.
Most of the users of theThermoFluidlibrary were able to build applica-tion oriented libraries on top ofThermoFluidin spite of the initially scarce state of the documentation. The efforts built on top of ThermoFluid had very different goals and perspectives. They range from feasibility stud-ies in the form of a masters thesis project to intensive industrial projects where several man-years of effort were spent on customized libraries.
Feedback from these projects has improved and is continuing to im-prove the usability of the library. A more widespread acceptance of object
Chapter 5. The ThermoFluid Library
oriented modeling usingThermoFluidfrom the current level would require three actions:
• Professional level documentation and training for new users.
• Commercial support and consulting services.
• Tools and specialized graphical user interfaces which are better ac-commodated to non-experts than the current Modelica tools.
Micro Turbine System
A micro turbine system for combined heat and electrical power generation is an industrial project that recently has been completed at the Swedish company Turbec AB. The combined heat and power system consists of the following main parts:
• Gas turbine engine and recuperator
• Electrical generator
• Electrical System
• Exhaust gas heat exchanger
• System for control and supervision
The basic compressor and turbine models were developed in a masters thesis project[Gómez ˜Pérez, 2001]. These models were later refined and adapted to the turbine model used at Turbec AB, [Haugwitz, 2002], who also modeled the control system and used ThermoFluidto model all auxil-iary system parts and heat exchangers. The exhaust gas heat exchanger is a gas-water counter-current flow type. It is assembled from library models for flue gas and water and did not need any user-written code.
One of the main uses of the model is to test control schemes. The model schematic in Figure 5.16 shows turbine, combustion chamber, recuperator, exhaust gas exchanger and turbine control system. As usual for turbines and compressors, a large part of the modeling effort has to be spent on the steady-state characteristics of the compressor and, to a smaller degree, the turbine.
In an islanding power configuration, where the micro turbine systems supplies a small electrical network in a stand-alone manner, the main goal of the control is to follow load changes as quickly as possible, with-out compromising the turbines lifetime by allowing turbine with-outlet tem-peratures to become too high. The electrical generator connected to the turbine generates high frequency three-phase AC. The AC is rectified and then converted to the standard electrical frequency of 50 Hz. This config-uration makes it possible to use minor variations in the turbine angular
5.10 ThermoFluid Applications
Figure 5.16 Schematic of a micro gas turbine system with recuperator and control system
velocity ω to store energy. This is an important feature, because slower transients result in lower value of the turbine outlet temperatures. Fig-ure 5.17 shows the results of a simulation with a serious of pulses to the demanded electrical power. The turbine outlet temperature is far from the critical limits and the variations in turbine angular velocity are well within acceptable limits.
CO2Refrigeration Cycles
A system simulation of future on-board cooling systems for airliners is currently developed in an ongoing research project of European Aeronau-tic Defense and Space Company (EADS) Airbus, Hamburg (Germany) and the Department of Technical Thermodynamics of the Technical Uni-versity Hamburg Harburg (TUHH). The aim of the project is a proof of concept of integrated cooling systems using the rediscovered refriger-ant CO2. Carbon dioxide was used as a refrigerant until the 1930s, but
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Turbine Temperature Turbine ω in rps
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Power Demand [kW]
Power Demand
Figure 5.17 Simulation results from applying demand power pulses to the micro turbine in an island power operation mode.
was then replaced by synthetical refrigerants that offered lower absolute pressures which in turn permitted simpler techniques and higher efficien-cies in conventional vapor compression systems. Due to the high global warming and ozone depletion potential of synthetical refrigerants, a sub-stitution of synthetical HCFCs is the aim of current research efforts, see [Pfafferot and Schmitz, 2002; Pfafferott and Schmitz, 2001].
The temperature and pressure at the critical point of CO2are 304.13K and 7.377M Pa. The refrigerant cycle has to be operated transcritically when the heat rejection takes place at an ambient temperature which is near or higher than the critical temperature. In aerospace applications, the heat rejection temperature is low in flight, resulting in a condensating mode, and high on the ground where the cycle operates transcritically.
The ThermoFluidlibrary contains high accuracy fluid property calcu-lations for CO2 published in [Span and Wagner, 1996] which are indis-pensable for detailed numerical studies of refrigeration processes. Unfor-tunately, the critical point singularity of such fluid property models make it impossible to use such published standards “as is” for simulations that pass the critical point, which is the case for transcritical CO2 cycles. Two alterations to the original formulation help to increase speed and
robust-5.10 ThermoFluid Applications
Centralised components
Supply Piping Remote components
Return
Control M device
Air RAM−
Expansion
Evaporator
Cooling Medium
Compressor
Receiver Internal heat exchanger
Gascooler
Figure 5.18 Schematic of a C O2refrigeration cycle with distributed cooling loads.
This configuration is used for on-board cooling systems of airplanes. Diagram used with permission from[Pfafferot and Schmitz, 2002].
ness of such calculations:
• Use of spline approximation to all properties on the phase bound-ary. These can be made very accurate depending on the number of interpolation points on the phase boundary. The speed-up obtained by using splines is substantial because it avoids phase equilibrium calculations.
• Move the critical point used in the numerical calculations to a slightly higher temperature (fractions of a Kelvin). This and the use of splines inside the 2-phase region avoids the singularity. The change in accuracy is negligible for system simulation and the calculations are robust because numerical failures at the critical point are avoided.
Pfafferott and co-workers developed a CO2-flow application library on top of ThermoFluid. They use base classes from ThermoFluid for all dy-namic model parts. Heat transfer and pressure drop correlations for the specific types of heat exchanger geometries used in CO2-cycles had to be developed. The models for evaporator, gas cooler, valves and pipes were assembled from partial models in theThermoFluidlibrary. Using the Ther-moFluid library allowed the developers to concentrate on the application specific model parts.
Figure 5.19 illustrates that system start-ups can be simulated without problems. A few seconds after switching on the system, the
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t = 60 sec.
t = 120 sec.
Initial
Compressor out Gascooler out
Internal HX out
Evaporator in
Evaporator out Compressor in
Figure 5.19 Steady state paths of two operating points of main components of a C O2refrigeration cycle after start-up. Diagram used with permission from[Pfafferot and Schmitz, 2002].
namic state of CO2crosses the critical point without numerical problems.
Steam Network Modeling
The Swedish company Solvina has used the ThermoFluid-library to de-velop a simulation model of the steam distribution network of the paper plant at Iggesund. The existing steam network was improved by adding two turbines, new control valves and pressure sensors. A new control sys-tem was also installed. The complete network with several boilers, tur-bines and a large steam network having four pressure levels was modeled from existing components in theThermoFluid-library. The model was used for two purposes:
• To build an operator training simulator, with LabView as a graphical user interface that mimics the real control system.
• To find suitable controller parameters for all important control loops in the steam net.
Both of these tasks contributed to an error-free start-up of the plant and an equally unproblematic operation since then. None of the simulated control loops needed tuning during start-up, see [Lindstrand, 2002].
5.10 ThermoFluid Applications
65 bar
12 bar
8 bar
3 bar 65 bar
12 bar
8 bar
3 bar
S im p lif ic a tio n
fuel
fuel Boiler Boiler
G5
G5 G4
G4
Figure 5.20 The steam net control system was extensively simplified with im-proved dynamic performance using the simulator. Figure courtesy of Solvina AB.
Analysis of the simulated control system revealed that the structure of the existing control system could be simplified significantly, as shown in Figure 5.20.
Fuel Cell Systems
Fuel Cell engines for power generation and automotive applications is a very active area of research and advanced development. Even though fuel cell technology has been used for many years, e. g., for combined power and
Chapter 5. The ThermoFluid Library
water supply for space applications, fuel cells have not yet reached a de-velopment status which allows them to be used in every-day applications.
TheThermoFluid-library has been used at United Technologies Research Center, Hartford, CT, to develop dynamic models of fuel cells including the complex fuel reforming unit. The reformer transforms hydrocarbons like natural gas or even gasoline to hydrogen and other byproducts. The hydrogen is used in the fuel cell to generate electric power.
TheThermoFluidlibrary was not designed with fuel cells as a prospec-tive application, but as a base library it offered models for most of the phenomena which are relevant for fuel cell modeling. The combination of the Modelica language, ThermoFluid-library and Dymola simulation envi-ronment was evaluated against a range of other possible modeling tools and techniques and was found to best fulfill all requirements of a flexible base to develop models for a range of different fuel cell products currently under development. The models are used both by members of the research department who develop the models of subsystems and libraries and by engineers at UTC Fuel Cells for development work. The fuel cell system models are the most complex systems built so far based on the basis of ThermoFluid. The models are also used for hardware-in-the-loop investi-gations of control system performance.