Dynamic modelling and simulation of the Graz Cycle for a renewable energy system

Critical atmospheric carbon dioxide levels and, in response, an increasing shift toward variable renewable energy sources causes the need of transient operation for thermal power plants coupled with carbon capture and storage. In this regard, dynamic process simulations are a valuable tool for predicting the cycling performance of power plants. In this study, the proposed Graz Cycle, an oxy-combustion power plant fired with natural gas, is investigated and modelled in (a) the steady-state design-point (i.e., full-load), (b) steady-state off-design (i.e., part-load), and (c) dynamic conditions. At full loads, the Graz Cycle power plant achieves a maximum net plant efficiency of 54.5%, comparable to the performance results of the competing Allam cycle found in relevant studies. In the off-design simulation, considering for the first time compressor component maps with variable inlet guide vanes, the load is reduced down to 50% achieving improved performance results compared to previous studies. The dynamic simulation boils down to a ramp rate analysis and shows that practical ramp rates of 5.55 %/min for load decrease and 18.18 %/min for load increase can be reached. The results show that the high ramp rates of the Graz Cycle enable balancing services to the electrical grid next to clean and dispatchable power generation. In this study, a systematic first-principle modelling approach has been developed with the final aim to evolve dynamic models for oxy-combustion carbon capture cycles, and a first-of-its-kind analysis of the dynamic operation of the Graz Cycle is presented.