A modern energy system based on renewable energy like wind and solar power inevitably needs a storage system to provide energy on demand. Hydrogen is a promising candidate for this task. For the re-conversion of the valuable fuel hydrogen to electricity a power plant of highest efficiency is needed. In this work the Graz Cycle, a zero-emission power plant based on the oxy-fuel technology, is proposed for this role. The Graz Cycle originally burns fossil fuels with pure oxygen and offers efficiencies up to 65% due to the recompression of about half of the working fluid. The Graz Cycle is now adapted for hydrogen combustion with pure oxygen so that a working fluid of nearly pure steam is available. The changes in the thermodynamic layout are presented and discussed. The results show that the cycle is able to reach a net cycle efficiency based on LHV of 68.43% if the oxygen is supplied "freely" from hydrogen generation by electrolysis. An additional parameter study shows the potential of the cycle for further improvements. The high efficiency of the Graz Cycle is also achieved by a close interaction of the components which makes part load operation more difficult. So in the second part of the paper strategies for part load operation are presented and investigated. The thermodynamic analysis predicts part load down to 30% of the base load at remarkably high efficiencies. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.