Gate cycle ®

GateCycle^® is a commercial heat balance software used to predict design and off-design thermodynamic performances of combined cycles, fossil boiler plants, cogeneration systems, combined heat-and-power plants, advanced gas turbine cycles, and many other energy systems. The software has a graphical interface and a wide library of components and auxiliaries typical of energy power units.

The software first appeared in 1981 and its development is carried on by GE Enter Software, a General electric Power Systems fully owned company. The database therefore features a wide set of GE components.

GateCycle^® is specifically designed to perform a large variety of analyses, such as:

analyzing an overall cycle for a proposed power system or cogeneration station. This analysis produces information on operating performance at all the state points throughout the plant, including overall cycle efficiency and power.

simulating the performance of existing systems at design and "offdesign" operating conditions.

predicting the effect of proposed changes or enhancements to existing plants.

analyzing advanced gas turbine designs, including designs that are fully integrated with the steam/water cycle.

design new equipment around existing equipment, as in repowered plants or for plant modification.

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mix and match existing compressors and turbines in advanced turbomachinery studies.

The software therefore appears as a tool strongly oriented to advanced analysis of power plants.

To build a model components are selected from the equipment list and dropped on to the model diagram. The process of building a model is typical of an icon based software and includes the dragging and dropping in the working environment of the main components of the plant, properly connecting the i/o ports and providing each component with a set of functional parameters. This last step appears to be one of the most problematic since a wide range of data are required for a correct identification of the component. These data may not always be easily available or accessible, especially when designing

“unconventional” units or for pre-feasibility studies when specific data sets of the components are still not available. This requires placing strong hypothesis or even guessing many of the parameters required.

Once everything is properly settled the program proceeds to calculate the steady state operating conditions compatible whit the boundary conditions imposed. Solution of the system is mathematically provided through an iterative process. The model will iterate through all of the equipment icons in the model until all energy and flow streams converge. This is achieved by calculating balance equations at each component while convergence is searched. The degrees of freedom for the software to find the solution are the characteristics dimensions of the components enclosed within the system. The software therefore designs all the main components of the system in such way to satisfy the imposed boundary conditions and required streams and to maximize the actual cycle efficiency. Design (or reference) cases therefore establish the operating characteristics (e.g. steam turbine design efficiency) and physical specifications (heat exchanger size) of key GateCycle equipment icons.

When an off design calculation is required the same optimal geometry calculated for on design conditions is kept and the cycle parameters are calculated for the new set of boundary conditions of streams.

The model therefore produces only a singular equilibrium point of operation for the given configuration of the plant proposed and results contain the state parameters of each component at steady rated conditions. By changing some of the parameters or the configuration new results can be obtained that describe the performances under steady state off design operating conditions. Results are provided in tabular form.

The GateCycle^® Graphical User Interface (GUI) enables easy building of any cycle and gives the user an almost unlimited flexibility in modeling and in displaying the calculation results [18]. A GateCycle^® model can be created by drawing the desired cycle configuration on the screen using the necessary equipment selected from a graphical menu of icon representations.

After the equipment icons have been positioned on the cycle diagram, the connections between the equipment icons can be drawn. The graphical tools provided with the ProVision interface automatically route the connecting streams on the diagram. Logic is provided in the connection procedures to ensure that all of the ports on each equipment icon are properly connected.

In the example that follows [19] a steam power plant has been considered with a biomass fired boiler.

The designed plant is represented through the GateCycle^® main operating window in Fig. 2.9. Off design conditions may evaluate changes in the actual fuel composition and changes in the environment temperature and humidity.

OVERVIEW ON MODELING OF THERMO-FLUID SYSTEMS

33 Fig. 2.9. GateCycle^® model of a biomass fired steam power unit [19].

Upon execution of the analysis module, the code first reads the connection and equipment icon data from the diagram and database. Next, the GateCycle^® code analyzes the connection data to determine the order of calculation for the equipment icons, a procedure known as 'flowsheet decomposition'. This procedure ensures that each equipment icon is included in the calculations and that the order selected will allow the iterative calculations to converge rapidly.

After determining the layout of the plant, the code reads in the system, equipment icon and macro data from the database. It checks for errors in the input values, sets defaults if necessary, and then continues on if no major errors have been found. The code then finds where pressures and flow rates are specified in the system, and checks whether these specifications are consistent throughout the cycle. The GateCycle^® code then proceeds to analyze the performance of the system by calling the appropriate equipment model routines one at a time in the order determined by the flowsheet decomposition procedure. After execution of each equipment model, the output data from that equipment icon is passed to all connected equipment icons. One system iteration is completed when all of the equipment models in the cycle have been executed. Macros are called where appropriate, depending on how they have been set up.

At the end of each system iteration, the GateCycle code uses a number of different criteria to determine if the model calculations have converged. First, the calculated output variables from each of the equipment icons must match the values from the previous system iteration within some numerical tolerance. Second, there must be a mass and energy balance around each of the equipment icons in the model and around the system as a whole; this calculation is possible since only steady state operation is possible then nor energy or mass storage has to be accounted for. Finally, the data for every outlet port must equal the data at the connected inlet port within the error tolerance. If there are macros in the model, they must also have converged within their specified tolerance. A typical GateCycle^® run will converge within two to forty system iterations, depending upon the complexity of the cycle being modeled, the

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convergence tolerances selected, the number and complexity of the macros, and the accuracy of the initial values in the database.

The software output is a report which contains the main figures of the system, as Fig. 2.10 for the system of Fig. 2.9. As an additional input/output interface to GateCycle^®, the CycleLink add-in is available. This Microsoft^® Excel^® based tool serves next to the GUI to allow for easy import/export of data between GateCycle^® and Excel^®. With CycleLink, an engineer can make customized interfaces to his own models. Since CycleLink uses Microsoft^® Excel^®, the GateCycle^® model becomes an integral part of customized Excel sheets, allowing further data analysis, including exergy calculations.

Fig. 2.10. GateCycle^® simulation report.

The calculations produced by the software therefore are provided for a specific operating condition and no transient and dynamical operating conditions can be evaluated.

Scientific literature proposes examples of the GateCycle^® software used to design a system at nominal operating conditions and the main parameters of the so designed units and components are employed within softwares that allow for dynamical simulation of the process in order to gather a deeper understanding of the interactions that the designed power generating unit may have with other interconnected units. In [20] for example a Rolls–Royce natural gas powered turbine (RB211T DLE) and its associated economizer is modelled in the simulator GateCycle^® in order to study the influence of some variables, such as the air and natural gas temperatures or the water circulation conditions, on the gas turbine and economizer performance. After the model has been completed successfully the interface variables were used as input of the dynamic model of the remaining integrated process, which include a

OVERVIEW ON MODELING OF THERMO-FLUID SYSTEMS

35 salt recrystallization process where the heat exchange between the cogeneration unit takes place thanks to four plate heat exchangers.

To be noted also that the software also does not allow to define customized components and the application therefore is limited to the field of conventional thermodynamic power units and not to general energy conversion systems. This characteristic, beside the fact that only steady state balance equations can be accounted for in the software, renders this tool unproper for the applications object of this study.