| dc.description.abstract | This thesis is devoted to the assessment of alternative concepts for improved gas turbine operation under varying loads in decarbonized energy systems. The aim is to contribute towards curbing the emission of Carbon Dioxide (CO2) per unit of power generated. Gas turbines, in this context, are tasked with the role of load-following units in which they often have suboptimal efficiency during part-load operation, as they adapt to the intermittent availability of renewable energy sources. The knowledge gap that underscores this study pertains to the untapped potential for improving the part-load efficiency of these decarbonized combined cycles when operating alongside renewable energy sources. The primary objective of this research is to unveil, implement and assess innovative solutions that will enhance the part-load efficiency of combined cycle gas turbines. The improvements made will contribute to diminishing CO2 emissions in scenarios involving natural gas combined cycles or contribute to cutting down fuel expenses for decarbonized combined cycles that utilize costly carbon-free fuels like hydrogen and ammonia. To achieve this, the study explores potential novel operation scenarios with decarbonized gas turbines, steam bottoming cycles, Organic Rankine Cycles (ORCs), and the combined heat and power cycles.
In the case of joint operation of ORCs and gas turbines, the part-load efficiency of the combined cycle is enhanced by implementing an ORC control strategy centered on Variable Area Nozzle (VAN) turbine technology. Through design and evaluation using an in-house tool developed in this study, the developed strategy demonstrates the ability to maintain part-load ORC efficiency close to the design values, outperforming other control strategies like sliding pressure, partial admission turbine, and throttling valve control. As a result, the combined cycle’s efficiency improves, leading to a reduction of 2.5 kilotons in annual CO2 emissions per gas turbine unit. Moreover, the compactness and autonomy of variable area nozzle turbines make them well-suited for offshore oil and gas installations. A design methodology for optimizing cycle power output while minimizing its footprint offshore is introduced. Moreover, a control strategy involving condensing pressure regulation is evaluated to further boost the thermal efficiency of the cycle with the developed VAN turbine control strategy. The results indicate that the proposed control strategy boosts the combined cycle efficiency further and successfully maintains part-load efficiency near design values and leads to a 5% reduction in annual CO2 emissions.
In the case of gas turbine operation with steam cycles, a solution is proposed to enhance part-load thermal efficiency through exhaust gas recirculation. By recirculating exhaust gas, which is known for its benefits for carbon-free combustion and carbon capture systems, the proposed strategy is evaluated for both single-shaft and two-shaft gas turbines operating. This approach eliminates the need to cool down the recirculated gas, reducing equipment footprint and making it well-suited for offshore applications. An in-house design and simulation tool is developed for assessing steam combined cycles with the presented gas recirculating systems, offering additional analysis flexibility in using carbonfree fuel mixtures. The resulting improvements in efficiency, emissions reduction, and fuel consumption are quantified, showcasing the effectiveness of the proposed solution.
An analysis of the transient behavior of a gas turbine operating with warm exhaust gas recirculation is presented. The effectiveness of a finely tuned control strategy integrated into the system is evaluated. The feasibility of implementing this control strategy in a gas turbine application is determined. The strategy effectively regulates exhaust gas temperature, ensuring stability of the temperatures, pressures, and shaft speed in the system. Results demonstrate an acceptable overshoot band and rapid response to set point changes, underscoring the control strategy’s efficacy and adaptability to dynamic operating conditions. | en_US |
