Thermodynamic modeling of a closed gas turbine process working with Helium and stoichiometric combustion of Hydrogen and Oxygen

For a successful transition of power generation away from fossil and nuclear fuels to renewable energy sources, the problems related to the storage of temporary surplus solar or wind energy and the subsequent utilization has to be solved. A promising approach for this is the generation of hydrogen from electrolysis, which can then be stored and used when necessary. However, large scale power generation from hydrogen with outputs in the Megawatt-range is currently difficult, as fuel cells do not offer such a large power output both from a technical and economical perspective and firing hydrogen in open cycle gas turbines will inevitably lead to high NOx-emissions, which is detrimental to the environment. This paper presents the results of a theoretical study of a closed cycle gas turbine working with helium and a stoichiometric combustion of hydrogen and oxygen, which is also a product of electrolysis. As helium is an inert gas, it is not affected by the combustion process, hence only water vapour is formed, which can be separated by condensation. As a consequence the process would allow large power outputs at zero emissions. For the study, different thermodynamic processes have been evaluated, starting from a simple closed cycle Brayton process to more sophisticated cycles with intercooling and humidification. For all processes, parameter studies have been performed to assess the sensitivity and impact of different parameters such as pressure ratio or turbine inlet temperature. In order to ensure the validity of the model, component efficiencies deduced from publications on the former Oberhausen II closed cycle helium gas turbine have been used. The results show that already the simple cycle process yields good efficiencies comparable to those of current state-of-the-art gas turbines. The improvement that can be achieved using process variations are considerably higher than for an open loop process, indicating that efficiencies in excess of 50% could be reached.