Dynamic exergy concept for more efficient transient operation of coupled thermal-electrical energy supply systems
Technical Thermodynamics
Final Report Abstract
The increasing share of fluctuating electricity feed-in from wind energy and photovoltaic systems as the energy transition progresses has a significant impact on the operating regime of conventional power plants. This results in reduced operating hours, more frequent production pauses, additional start-up and shutdown processes and a sharp increase in load changes, resulting in a much more dynamic operation mode overall. As frequent load changes have not been the focus of optimizations in the past, potential for improving transient operating behavior is suspected. For their investigation and evaluation, exergy analyses can be used to quantify the actual thermodynamic losses in energy conversion processes. In order to conduct a transient exergy analysis of the combined-cycle power plant serving as the object of investigation at component level, a comprehensive, dynamic simulation model was developed that includes the process components such as the waste heat boiler and watersteam cycle as well as the control system. The model was developed in the modelling language Modelica using the ClaRa component library, which was expanded in the scope of the work to include the calculation of the exergetic balance and state equations, whereby the required exergy variables of all spatially zero- and one-dimensionally discretized components can be determined. The analyses carried out with the simulation model have shown that in transient operation, heat transfer with increased temperature differences and storing and withdrawing exergy in the various subsystems are the main causes of increased exergy destruction. Lower load change rates and an increasingly quasi-stationary mode of operation generally increase the exergy efficiency of the process. Based on the results of the analyses, measures to improve exergetic efficiency in transient operation were investigated. This includes modifications to the control of the feedwater tank pressure and the steam drum level, such as adjustment of controller parameters, asymmetrical control, feedforward control and load-dependent set point control. Due to various compensation effects, the measures investigated have only a very small influence on the exergy efficiency. The maximum increase in exergetic efficiency achieved is 0.04 percentage points. The application of the developed methodology to the model of a lignite-fired power plant confirmed the observations. For the combined cycle plant, different load change modes in three-block operation were also investigated, which in the most favorable case show an exergetic efficiency that is 0.1 percentage points higher. In the present thesis, it was found that no significant increase in exergetic efficiency can be achieved in transient operation by adjusting the control technology, but that there is potential for improving transient modes of operation.
Publications
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Evaluation of Flexibility Optimization for Thermal Power Plants. Volume 2: Heat Exchanger Technologies; Plant Performance; Thermal Hydraulics and Computational Fluid Dynamics; Water Management for Power Systems; Student Competition. American Society of Mechanical Engineers.
Hübel, Moritz; Prause, Jens Hinrich; Gierow, Conrad; Hassel, Egon; Wittenburg, Raphael & Holtz, Dorian
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Effects of rising dynamic requirements on the lifetime consumption of a combined cycle gas turbine power plant. Energy Procedia, 158, 5717-5723.
Wittenburg, Raphael; Hübel, Moritz; Prause, Hinrich; Gierow, Conrad; Reißig, Martin & Hassel, Egon
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„Dynamisches Exergiekonzept für eine effizientere transiente Betriebsweise gekoppelter thermisch-elektrischer Energieversorgungsstrukturen“. Fachvortrag auf dem VDI Thermodynamik-Kolloquium. Duisburg
R. Wittenburg, M. Hübel & E. Hassel
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„Einsatzplanung eines Gas- und Dampfturbinenkraftwerks mittels instationärem Prozessmodell“. In: Kraftwerkstechnik 2020. Hrsg. von M. Beckmann und A. Hurtado. Freiberg: SAXONIA Standortentwicklungs- und -verwaltungsgesellschaft mbH
J. H. Prause, R. Wittenburg, D. Holtz, E. Hassel & C. Gierow
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Transient Exergy Analysis of the Dynamic Operation of a Combined Cycle Power Plant. ASME 2021 Power Conference. American Society of Mechanical Engineers.
Wittenburg, Raphael; Hübel, Moritz; Holtz, Dorian & Müller, Karsten
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„Exergieanalyse des transienten Betriebsverhaltens eines GuD-Heizkraftwerks“. In: Kraftwerkstechnik 2021. Hrsg. von M. Beckmann und A. Hurtado. Freiberg: SAXONIA Standortentwicklungs- und -verwaltungsgesellschaft mbH
R. Wittenburg, K. Müller, D. Holtz & M. Hübel
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„Transiente Exergieanalyse und Verbesserung des dynamischen Betriebs eines GuD-Kraftwerks“. Fachvortrag auf dem VDI Thermodynamik-Kolloquium
R. Wittenburg, K. Müller & D. Holtz
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„Exergetische Bewertung von Maßnahmen zur Primärregelleistungserbringung in Kraftwerken“. Fachvortrag auf dem VDI Thermodynamik-Kolloquium. Chemnitz
R. Wittenburg, T. H. Pruß, D. Holtz & K. Müller
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Transition of district heating from fossil to renewable energies – Pathways analysed by dynamic simulation. Renewable Energy Focus, 45, 271-286.
Wittenburg, Raphael; Gierow, Conrad; Pötke, Rasmus; Müller, Karsten & Holtz, Dorian
