Dynamic modelling and simulation of power plants with Apros

The selected hybrid configuration is dynamically modelled and simulated with Apros software. Dynamic modelling and simulation (DMS) is needed for the development of CSP and conventional power plant hybrids, as the solar irradiation is a fluctuating ener-gy source, which impacts to the behaviour and dynamics of the conventional steam power plant can be investigated with dynamic simulations. DMS aims to predict dynam-ic behaviour of power plants and provide a virtual tool, whdynam-ich can be operated similarly than the actual power plants. The major uses of plant scale DMS can be categorized into six groups: development of control strategies, analysis of the system operation, verifica-tion of plant design, testing of control system, training of operators and development of operation and control practises. However, before DMS is conducted, it often needs to be complemented with data from steady state simulations, various dimensioning

calcula-tions and computational fluid dynamics (CFD). These provide input data, for example, of process connections, physical dimensions and positions of process equipment and pipelines, equipment specific parameters, automation concept diagrams, control pa-rameters and initial condition information. (Lappalainen et al. 2012, p.62-63)

One of the available DMS tools is Advanced Process Simulator (Apros) software, which is multifunctional software for the DMS of different industrial processes including their automation and electrical systems. The software has been developed since 1986 by For-tum and VTT Technical Research Centre of Finland, and it is committed to continuous development. Currently, it is used in 26 countries (Apros 2015a) for multiple applica-tions, such as nuclear and combustion power plants, pulp and paper mills, general heat-ing and coolheat-ing processes, smart cities and alternative power generation applications, such as fuel cells and solar power. Thus, Apros can be used for the purpose of operation and maintenance (O&M), engineering, and research and development (R&D) (Figure 43) in various industrial processes. (Apros Training Course Material 2015)

Figure 43. Use of Apros software (Apros Training Course Material 2015).

Apros uses a fully graphical user interface, which allows user to enter process-related input data through a dialog window to components and connections between them. The components can be dragged and dropped from comprehensive model libraries, which cover component modules, such as pipes, valves, pumps, tanks, heat exchangers, tur-bines, measurements, proportional-integral-derivative controllers (PID-controllers) and electric generators. The component modules are analogous with the concrete devices.

As the user draws connection between the component modules and inserts input data to the model, the result is a piping and instrumentation diagram with specific additions considering the simulation, such as supervision of the calculated variables (Figure 44).

Apros is an online simulator, which allows changes in the configuration during simula-tions. (Apros 2015b) In addition, Apros is an open simulator, which allows user to in-clude own models and have an easy access to external models, control room equipment and automation systems (Apros Training Course Material 2015).

Figure 44. Graphical user interface of Apros. Figure is taken from an exercise per-formed in the Apros Training course.

Apros includes a systematic model hierarchy, which consists of diagrams, process com-ponent modules and calculation level (Figure 45). A single model consists of multiple diagrams, a single diagram consists of multiple component modules, and a single com-ponent module consists of calculation levels, such as nodes and branches. The user manages the model with diagrams, which can be either generic, process or automation type. However, the model is built with component modules. In addition, a process dia-gram can only include process component modules, and an automation diadia-gram can only include automation component modules, whereas generic diagram can include both component types. A component module consists of calculation levels, which include the necessary model equations and choice of solution methods. Apros creates automatically the calculation level for component modules, and the user has to seldom change the details in the calculation level. One example of calculation level of a process with two points and a heat pipe is exposed in the Appendix I. The state variables, such as pres-sure and enthalpy, are calculated in the nodes and variables considering the flow are calculated in branches, which act as borders for nodes. The simulated fluid can be se-lected by the attribute fluid section, which determines the properties and composition of the fluid. The options are, for example, air, flue gas, water-steam and combustion fuels.

Furthermore, it is possible to define own fluid in Apros. (Apros Training Course Mate-rial 2015)

Figure 45. The model hierarchy in Apros and example of process and calculation level (Apros Training Course Material 2015).

In the calculation level, Apros uses flow model, which refers to the selection of thermo-hydraulic solution. In other words, it refers to the selection of accuracy level in the solu-tions of flows, pressures and heat transfer. Different thermohydraulic flow models can be used in different components and connection modules of a simulated system. The options for the flow model are 0, 1, 2, 5 and 6, from which the most commonly used ones are 2 and 6 models. The flow model 2 is used as default, and it is called as 3-equation model, since it resolves conservation 3-equations for mass, momentum and ener-gy for a homogenous fluid mixture of liquid and gas. In other words, it includes the so-lution of thermohydraulic node pressures, flows and enthalpies and the simulations of the heat structure temperatures and the heat transfer between the fluid and heat struc-tures. All available fluids for simulations are compatible with the flow model 2. The flow model 6 is called 6-equation model, since it calculates the three conservation equa-tions separately for liquid and gas phases. Compared to the flow model 2, the heat trans-fer is also simulated between the two phases. In addition, the only available fluid for flow model 6 is water-steam mixture. (Apros Training Course Material 2015)

The flow model 5 is a modification of the flow model 6 with a difference in calculation of momentum, as it is calculated for a homogenous mixture instead of two phases, and it uses explicit solution. It is recommended to use the flow model 6 instead of the flow model 5. The flow model 1 differs from flow model 2, as the pressures, flows and en-thalpies are calculated using simplified conservation equations and less material proper-ties with no iteration of the process. It can be used in processes, where is no need for simulation of heat transfer between the fluid and heat structures. In the flow model 0, the mass flows in the pipelines are given by user and not resolved from pressure dynam-ics. Instead, the dynamics of the process reside in tank modules, as the solution