Hydrogen is seen as one of the most promising zero carbon emission fuels for gas turbines and combined cycle installations. Hydrogen can be used as energy storage to balance renewable energy by converting water into hydrogen and oxygen during periods with excess renewable and applying the hydrogen in a fuel cell or thermal power plant during periods of shortage. With the efficiencies of modern CCGT plants of around 60% and the project efficiency of elektrolysers of 80%, a total round trip electrical efficiency of around 50% can be achieved. A significant improvement in efficiency for the conversion of hydrogen into power upto 70%-75% can be achieved by using the closed Brayton/Rayleigh cycle using steam as a working fluid and hydrogen and pure oxygen as the energy input. These H2/O2 cycles will cooperate well with a system where elektrolyzer provide both the hydrogen and oxygen.

This paper studies potential improvement of the H2/O2 cycle. Several cycles from literature are compared based upon an exergy analysis and the best element of all cycles are combined. The main modifications versus the Graz H2/O2 cycle are the incorporation of a reheat combustor and the application of a 1700  C inlet temperature cooled high pressure turbine. The high inlet temperature of the HP turbine is key to achieve the high cycle efficiency. The optimum cooling of the HP turbine is therefore studied in more detail. The new modified cycle can achieve an efficiency of 75.65 %, an improvement of 5.2% versus the original Graz H2/O2 cycle.

The exergy analysis shows that with the modified cycle the main losses occur in the combustor.  The addition of a set of SOFC fuel cells to reduce the exergy losses at an operating temperature of 1000 C is studied. The fuel cell can work in it’s ideal conditions: pressurized with pure hydrogen as a fuel and pure oxygen as a oxydizer leading to high fuel utilization. Combustor exergy losses decrease significantly when the ingoing streams are preheated to a 1000 degree. The maximum turbine inlet temperature of 1700 degree is achieved by the afterburner. With the addition of the fuel cell, efficiency upto 85% can be achieved depending on the configuration and assumed losses

 


The Optimization of Hydrogen Oxygen Cycles

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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