The design of an innovative hydrogen-oxygen power system based upon a solid oxide fuel cell and an advanced thermodynamic cycle

Abstract

In the battle against climate change, humankind needs to develop alternative energy systems if it wants to survive on this planet. Fossil fuels, polluting the air with CO2, must make way for renewable sources so that climate change is put to a stop. Several new energy initiatives based on renewable sources are deployed worldwide, these however are mostly for electricity generation. There are elatively few zero emission alternatives developed for the energy demand from the transport sector, therefore, new small scale energy systems need to be developed in order to achieve the goals stated in the Paris agreement. The Hydrogen Roadmap” is a vital part of the European Union in achieving the Paris climate agreement, which uses hydrogen for future energy generation. Gas turbines, steam turbines and fuel cell applications are investigated thoroughly for small scale energy systems throughout the years. Fewer research is conducted about the integration of these three energy technologies and specially sparse research has been conducted in the area of pure hydrogen oxygen combustion for these energy systems, resulting in opportunities for development. This report introduces an alternative small scale power system suited for the newest inland shipping vessels. It proposes a basic design for a 3 MW SOFC integrated hydrogen oxygen fired combined power cycle, feasible within 10 to 20 years of research and development. First, the theoretical background for the research is elaborated covering the thermodynamics of traditional power cycles, exergy analysis theory and zero emission cycles such as hydrogen oxygen fuelled power cycles. The electrochemical research containing the fuel cell operating specifics and the advantages of integrating fuel cell stacks in power cycles is introduced next. The theoretical background ends with an elaboration on the preliminary design steps for power cycle components regarding turbomachinery and heat exchange equipment. After the relevant subjects regarding power cycles and fuel cells are elaborated, a basic cycle was designed. From literature, several starting points such as the maximum turbine inlet temperature and inlet temperatures of the SOFC were defined. With a few determining thermodynamic states known on beforehand, a pressure and temperature analysis was carried out to determine the limiting operating conditions and the remaining thermodynamic states in the cycle. The cycle was eventually designed as a SOFC integrated Brayton Rankine cycle coupled via a single pressure heat recovery steam generator, where the SOFC is situated upstream of the combustor in the Brayton part of the cycle. All components in the cycle are evaluated using an exergy analysis, from which the final exergetic efficiency was determined as 73.09%. From the exergy analysis of the proposed basic cycle became clear that there was potential for improvement regarding the HRSG design. The improved cycle is therefore designed with a dual pressure HRSG, while the rest of the combined cycle remains equal compared to the basic cycle. With the extra high pressure regime, a high pressure turbine is added, thus creating an extra point in the system from which useful power is extracted from the medium. The exergetic efficiency increase of the HRSG alone amounts to 0.74%, but due to extra compressor and pump work and losses, the improved cycle results in a total efficiency of 73.47%, implying an increase of 0.38% compared to the basic cycle. To give a first practical indication about the size and specifics of the proposed cycles, a preliminary design of the components is made. The turbomachinery is sized according to the appropriate non dimensional numbers, the specific speed and diameter. An indication about the SOFC stack size and its operating specifics are given, together with the preliminary designs of the sensible and latent heat exchangers. A total preliminary size estimation of the cycle predicted a system with the size of approximately 4.5 shipping containers.