Dynamic modeling and simulation of steam power plants is often adopted as a tool for control design, personnel training, efficiency improvement and on-line diagnostic. The boiler is possibly the most complex component of the thermal power plant. A usual boiler configuration is the so-called Once-Through arrangement. A common problem in 2-phase systems modeling is the correct calculation of the phase boundary. This is technically interesting in such boilers: the location of the phase transition changes rapidly depending on load conditions and temperature distribution along the walls. A lumped parameters, one-dimensional evaporator model implementing a moving boundary approach is presented and first validation results are discussed. The model takes into account the influence of radiation and convection on the gas side. The flow inside the pipes is divided into 3 regions (sub-cooled, 2-phase, superheated) and the model calculates the locations of the 2-phase transitions and the average steam quality along the pipes. The system is discretized using a staggered grid for higher numerical stability and is implemented in the computer program Aspen Custom Modeler (ACM). Results include the calculation of the system response to input signals simulating a load variation and a validation by comparison with a model implemented in a commercial software for power plant simulations (MMS). Input data, parameters and geometry are taken from an existing plant operating in Uppsala, Sweden.
The high-temperature gas-cooled reactor (HTGR) is a nuclear reactor which can be designed to be inherently safe. When the helium heated in the reactor is used directly in a closed-cycle gas turbine system, the excellent safety characteristics can be combined with a high efficiency. Because of the small scale and simple modular design (both a result of the inherent safety) the HTGR is suitable for combined heat and power (CHP) production. In this article an energy conversion system for a nuclear gas turbine cogeneration plant is designed. First, with help of a basic model, a cycle choice is made and some parameters such as pressure ratio and heat-power ratio are optimized. With a steady state model a conceptual design is made and a sensitivity analysis is carried out for a number of design parameters. Finally, the off-design behaviour is optimized.
Using the dynamic model of the cogenerating nuclear gas turbine plant developed in Part I of this article, the dynamic behavior of this plant is analyzed and a control structure is designed. First it is determined how several design choices affect the system dynamics. Then the requirements and options for a control system design are investigated. A number of possible control valve positions in the flowsheet are tested with transients in order to make an argued choice. The model is subsequently used to determine the optimal working conditions for different heat and power demands, these are used as set-points for the control system. Then the interaction between manipulated and controlled variables is mapped and based on this information a choice for coupling them in decentralized feedback control loops is made. This control structure is then tuned and tested. It can be concluded that both heat and power demand can be followed with acceptable performance over a wide range.
The high-temperature gas-cooled reactor is a promising concept for inherently safe nuclear power generation. This article deals with dynamic modeling of a combined heat and power plant, based on a helium-cooled reactor in combination with a closed-cycle gas turbine system. A one-dimensional flow model describing the helium flow and the two-phase water flow is used through the whole plant, with different source terms in different pieces of equipment. A stage-by-stage model is produced for the radial compressor and axial turbine. Other models include the recuperator, water/helium heat exchangers, a natural convection evaporator, valves, etc. In Part II the model will be used to analyze the dynamic behavior and to design a control system.
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